The majority of weather radar systems in operation today utilize a single polarization strategy to enhance precipitation reflectivity. Liquid hydrometeors (e.g. raindrops) deviate from a sphere when their radius is greater than about 1 mm and have a shape more like that of an oblate spheroid with a flattened base (similar to a hamburger bun) that gives a slightly stronger horizontal return. Hence, current radar systems are typically horizontally polarized to enhance precipitation returns.
However, singly polarized radar systems have severe limitations in regions with partial beam blockage and such systems do not facilitate hydrometeor classification. To overcome these shortcomings of singly polarized weather radar systems, systems with alternating pulses of horizontally and vertically polarized signals have been developed. These dual polarized radar system, sometimes referred to as “polarimetric weather radars,” offer several advantages over conventional radars in estimating precipitation types and amounts. Foremost among these advantages are the capability to discriminate between hail and rain, detect mixed phase precipitation, and estimate rainfall volume.
Current dual polarized radar systems utilize polarization that is altered sequentially between linear vertical and linear horizontal to capture data enhancing values, such as, for example: (1) reflectivity factors at both horizontal and vertical polarization; (2) differential reflectivity for two reflectivity factors; (3) cumulative differential phasing between the horizontally and vertically polarized echoes; (4) correlation coefficients between vertically and horizontally polarized echoes; and (5) linear depolarization ratios. In addition, Doppler velocity and spectrum width can be obtained by suitably processing the horizontally and vertically polarized return signals.
Dual polarized radar systems also allow for the implementation of precipitation classification schemes from inference radar processing of hydrometeor shapes as discussed in various papers authored by practitioners who work in these areas, such as, Ryzhkov, Liu, Vivekanandan, and Zrnic. In addition, by looking at phase differences between the horizontal and vertical components, the effects of partial beam blockage can be mitigated and greater clutter rejection can be obtained. However, the underlying assumption is that subsequent pulses (those of each polarization) are highly correlated and provide an effective velocity range reduced by a factor of two.
Another limitation of current alternating dual polarization radar systems is long dwell times and velocity range reductions. Any received reflection signal resulting from either polarization modes is assumed to come from the same scatterers (e.g. hydrometeors). In order to correlate the data from both the horizontally polarized and vertically polarized channels in current systems utilizing a waveguide switch, a single polarization pulse is transmitted followed by a period of delay (the dwell time) while reflections signals are being received. The opposing polarity pulse is subsequently sent and additional data is received by the same (single) receiver chain during a second dwell time. Reception of reflection signals, therefore, occurs during these two dwell periods during antenna rotation within a single beamwidth, resulting in a longer total dwell time for each beamwidth interrogation. Similarly, since the dwell time for each beamwidth interrogation (vertical+horizontal) is doubled, computational velocity perception is halved, thereby limiting the ability of current systems to resolve relatively high wind velocities in radar returns.
Improved dual polarization weather radar systems use simultaneous dual polarization modes to solve issues such as long dwell times and velocity range reductions instead of alternating polarization modes. Dual polarized systems, which propagate both a horizontal and a perpendicularly vertical wave simultaneously, have additional problems relating to the interference between the horizontal and vertical component. As shown in FIG. 1A, a dual polarized simultaneous wave 10 has a horizontal component 12 and a vertical component 14. The two components are characterized by their amplitudes and the relative phase between them. When viewed along the direction of propagation, the tip of propagated wave vector of a fully polarized wave traces out a regular pattern of an ellipse. The shape of the ellipse is governed by the magnitudes and relative phase between the horizontal 12 and vertical components 14 of the wave. As the dual polarized elliptical wave 10 hits a reflective surface, the reflective surface can change the polarization of the wave 10 as it is reflected to be different from the polarization of the wave as it propagates. The radar antenna may be designed to receive the different polarization components of the wave 10 simultaneously. For example, the H and V (horizontal and vertical) parts of an antenna can receive the two orthogonal components of the reflected wave.
A radar system using H and V linear polarizations can thus have the following signals (or channels): HH—for signals that are horizontal transmit and horizontal receive, VV—for signals that are vertical transmit and vertical receive, HV—for signals that are horizontal transmit and vertical receive, and VH—for signals that are vertical transmit and horizontal receive. The HH and VV combinations are referred to as like-polarized, because the transmit and receive polarizations are the same. The HV and VH channels are cross-polarized because the transmit and receive polarizations are orthogonal to one another. The cross-polarized signals are created from a reflection that is not aligned perpendicularly with the direction of the propagation. As the reflection returns to the antenna at an angle other than parallel to the propagated angle, a portion of the horizontal signal results in a vertical component, and vice versa.
By examining the four signals, HH, VV, HV, and VH, all of the information necessary to describe the reflective source is captured. By examining the relative angle between the different signals and the power of the signals, the reflective source may be identified. The dual polarized radars of today, however, are unable to isolate and capture the different signals if the horizontal and vertical propagating signals are sent simultaneously.